Generally, in a manufacturing process of a semiconductor device, fine patterns are formed by using photolithography. Furthermore, several sheets of transfer masks (photomasks) are generally used for formation of those fine patterns. Generally, a transfer mask is produced by providing fine patterns formed of a metallic thin film or the like on a transparent substrate. Photolithography is also used in a process of manufacturing a transfer mask.
For manufacturing a transfer mask by using photolithography, a mask blank having a thin film (e.g., a light-shield film) is used to form a transfer pattern (mask pattern) on a transparent substrate such as a glass substrate.
The production of a transfer mask using a mask blank includes an exposure process of drawing a desired pattern on a resist film formed on a mask blank, a development process of developing the resist film according to the desired pattern being drawn to thereby form a resist pattern, an etching process of etching a thin film according to the resist pattern, and a process of separating and removing the remaining resist pattern.
In the aforementioned development process, a desired pattern is drawn (exposed) on the resist film formed on the mask blank, and a developing solution is then supplied to dissolve a portion of the resist film that is soluble in the developing solution. Thus, a resist pattern is formed. Furthermore, in the aforementioned etching process, an exposed portion of the thin film on which no resist pattern is formed is dissolved by dry etching or wet etching with use of a mask of the resist pattern. Thus, a desired transfer pattern is formed on a transparent substrate. In this manner, a transfer mask is produced.
Not only a finer transfer pattern formed in a transfer mask, but also a shortened wavelength of an exposure light source used for photolithography is required in order to form a finer pattern of a semiconductor device. In recent years, the wavelength of exposure light sources used for manufacturing a semiconductor device has gradually been shortened from a KrF excimer laser (with a wavelength of 248 nm) to an ArF excimer laser (with a wavelength of 193 nm).
Furthermore, a half-tone type phase shift mask has been known as a transfer mask other than a conventional binary mask having a light-shield film pattern formed of a chromium-based material on a transparent substrate. The half-tone type phase shift mask has a semitransparent film (half-tone type phase shift film) on a transparent substrate. The semitransparent film permits light having an intensity that does not substantially contribute to exposure to pass therethrough (for example, 1% to 20% to the exposure wavelength) and provides a predetermined phase difference to the transmitted light. For example, a material containing a molybdenum silicide compound or the like is used for the semitransparent film. The half-tone type phase shift mask includes a semitransparent part in which a semitransparent film has been patterned and a transparent part that has no semitransparent film and permits exposure light to pass therethrough. With the semitransparent part and the transparent part, the phase of light transmitted through the semitransparent part substantially has an inversed relationship with respect to the phase of light transmitted through the transparent part (i.e., the phase is shifted). Thus, light rays that have passed near a boundary portion between the semitransparent part and the transparent part and entered the counterpart area due to a diffraction phenomenon cancel out, so that the light intensity becomes nearly zero at the boundary portion. Thus, the half-tone type phase shift mask improves the contrast, i.e., the resolution at the boundary portion.
Furthermore, transfer patterns become finer in the exposure technology using exposure light of an ArF excimer laser (with a wavelength of 193 nm). Therefore, it is necessary to cope with a line width of a pattern that is shorter than the wavelength of the exposure light. Thus, there have been developed super-resolution technology, such as an oblique illumination projection method and a phase shift method, and ultrahigh-NA technology (immersion lithography or the like) where NA (Numerical Aperture)=1 or more.
As transfer patterns become finer, the width of resist patterns becomes narrower. Therefore, the aspect ratio becomes high with the film thickness of a conventional resist film. Therefore, it is getting difficult to form a transfer pattern by dry-etching a light-shield film with use of a mask of the resist pattern.
As one of solutions for this problem, there has been developed a binary mask blank in which a light-shield film is formed of a film containing a transition metal and silicon so that the light-shield film can be dry-etched with a fluorine-based gas and in which an etching mask film of a chromium-based material is formed on the light-shield film (Patent Document 1). A method of producing a transfer mask from this mask blank includes first forming a resist pattern on the etching mask film, dry etching with an etching gas of a mixture gas of chlorine and oxygen while using a mask of the resist pattern, and forming a transfer pattern in the etching mask film. Then dry etching is conducted with an etching gas of a fluorine-based gas while using the transfer pattern of the etching mask film as a mask, thereby forming a transfer pattern in the light-shield film. Thereafter, the etching mask film is removed. A binary transfer mask is produced through a predetermined cleaning process.